A Global Navigation Satellite System (GNSS) is a navigation system that makes use of a constellation of satellites orbiting the earth to provide signals to a receiver on the earth that computes its position on the earth from those signals. Examples of such satellite systems are the NAVSTAR Global Positioning System (GPS) deployed and maintained by the United States, the GLONASS system deployed by the Soviet Union and maintained by the Russian Federation, and the GALILEO system currently being deployed by the European Union (EU).
Each GPS satellite transmits continuously using two radio frequencies in the L-band, referred to as L1 and L2, at respective frequencies of 1575.41 MHz and 1227.60 MHz. Two signals are transmitted on L1, one for civil users and the other for users authorized by the Unites States Department of Defense (DoD). One signal is transmitted on L2, intended only for DoD-authorized users. Each GPS signal has a carrier at the L1 and L2 frequencies, a pseudo-random number (PRN) code, and satellite navigation data. Two different PRN codes are transmitted by each satellite: a coarse acquisition (C/A) code and a precision (P/Y) code which is encrypted for use by authorized users. A GPS receiver designed for precision positioning contains multiple channels, each of which can track the signals on both L1 and L2 frequencies from a GPS satellite in view above the horizon at the receiver antenna, and from these computes the observables for that satellite comprising the L1 pseudorange, possibly the L2 pseudorange and the coherent L1 and L2 carrier phases. Coherent phase tracking implies that the carrier phases from two channels assigned to the same satellite and frequency will differ only by an integer number of cycles.
Each GLONASS satellite transmits continuously using two radio frequency bands in the L-band, also referred to as L1 and L2. Each satellite transmits on one of multiple frequencies within the L1 and L2 bands respectively centered at frequencies of 1602.0 MHz and 1246.0 MHz. The code and carrier signal structure is similar to that of NAVSTAR. A GNSS receiver designed for precision positioning contains multiple channels each of which can track the signals from both GPS and GLONASS satellites on their respective L1 and L2 frequencies, and generate pseudorange and carrier phase observables from these. Future generations of GNSS receivers will include the ability to track signals from all deployed GNSSs.
The purpose of an AINS is to compute navigation data comprising vehicle position, velocity, acceleration, orientation (e.g., roll, pitch, heading) and angular rate via the combination of an inertial navigation system (INS) and aiding navigation sensors. A GNSS-aided INS uses one or more receivers capable of receiving and processing signals from one or more GNSS's as an aiding sensor. GNSS-AINS has been successfully demonstrated as an accurate source of position and orientation information for various survey applications from a moving platform. One of the most significant achievements in recent years is the successful demonstration and subsequent deployment of a GNSS-AINS for direct georeferencing of aerial photogrammetry images. Other applications include mobile mapping/survey from a land vehicle and sea floor bathymetry from a survey vessel.
To achieve accurate positioning of a mobile platform with a GNSS-AINS, relative or differential positioning methods are commonly employed. These methods use a reference GNSS receiver located at a known position, in addition to the data from the INS and the rover GNSS receiver (both on the mobile platform), to compute the position of the mobile platform relative to the reference receiver. The most accurate known method uses relative GNSS carrier phase interferometry between the rover and reference GNSS antennas plus resolution of integer wavelength ambiguities in the differential phases to achieve centimeter-level positioning accuracies. These differential GNSS methods are predicated on the near exact correlation of several common errors in the rover and reference observables. They include ionospheric and tropospheric signal delay errors, satellite orbit and clock errors, and receiver clock errors.
When the baseline length between the mobile platform and the reference receiver does not exceed 10 kilometers, which is normally considered a short baseline condition, the ionospheric and tropospheric signal delay errors in the observables from the rover and reference receivers are almost exactly the same. These atmospheric delay errors therefore cancel in the rover-reference differential GNSS observables, and the carrier phase ambiguity resolution process required for achieving centimeter-level relative positioning accuracy is not perturbed by them. If the baseline length increases beyond 10 kilometers (considered a long baseline condition), these errors at the rover and reference receiver antennas become increasingly different, so that their presence in the rover-reference differential GNSS observables and their influence on the ambiguity resolution process increases. Ambiguity resolution on single rover-reference receiver baselines beyond 10 kilometers becomes increasingly unreliable. This attribute limits the mobility of a GNSS-AINS with respect to a single reference receiver, and essentially makes it unusable on a mobile mapping platform that covers large distances as part of its mission, such as an aircraft.
A network GNSS method computes the position of a rover receiver using reference observables from three or more reference receivers that approximately surround the rover receiver trajectory. This implies that the rover receiver trajectory is mostly contained by a closed polygon whose vertices are the reference receiver antennas. The rover receiver can move a few kilometers outside this polygon without significant loss of positioning accuracy. A network GNSS algorithm calibrates the ionospheric and tropospheric signal delays at each reference receiver position and then interpolates and possibly extrapolates these to the rover position to achieve better signal delay cancellation on long baselines that could be had with a single reference receiver. Various methods of signal processing can be used, however they all yield essentially the same performance improvement on long baselines. As with single baseline GNSS, known GNSS solutions are still inadequate for a mobile mapping platform that covers large distances as part of its mission, such as an aircraft.
Another problem associated with mobile mapping/survey applications is an insufficiently fast recovery of positioning accuracy after a loss of the rover GNSS signal. The typical time to recovery of reliable precise positioning accuracy is 15-60 seconds, depending on the number of observables and their geometry used in the position solution. Such signal outages tend to occur on an aircraft engaged in a survey mission when the aircraft executes rapid high bank-angle turns (“sharp turns”) from one survey line to the next. Sharp turns between survey lines provide the most economical execution of a survey mission. Typical survey trajectories include many parallel survey lines joined by 180-degree turns. Consequently these sharp turns and resulting signal outages can occur frequently. Previous GNSS-AINS implementations for this application required the pilot to fly low bank angle turns (“flat turns”) to maintain the rover GNSS antenna orientation toward the sky and thereby avoid GNSS signal loss. Such flat turns required significantly longer times to execute than sharp turns, resulting in additional aircraft operation expenses.
FIG. 1 shows a known architecture for an AINS. The IMU 1 generates incremental velocities and incremental angles at the IMU sampling rate, which is typically 50 to 500 samples per second. The corresponding IMU sampling time interval is the inverse of the IMU sampling rate, typically 1/50 to 1/500 seconds. The incremental velocities are the specific forces from the IMU accelerometers integrated over the IMU sampling time interval. The incremental angles are the angular rates from gyroscopes in the IMU 1, integrated over the IMU sampling time interval. The inertial navigator 2 receives the inertial data from the IMU and computes the current IMU position (typically latitude, longitude and altitude), velocity (typically North, East and Down components) and orientation (roll, pitch and heading) at the IMU sampling rate.
The aiding sensors 5 are any sensors that provide navigation information that is statistically independent of the inertial navigation solution that the INS generates. Examples of aiding sensors are one or more GNSS receivers, an odometer or distance measuring indicator (DMI), and a Doppler radar velocity detector.
The purpose of the Kalman filter 4 in the AINS configuration is to estimate the errors in the inertial navigator mechanization and the inertial sensor errors. The Kalman filter 5 does this by comparing the INS navigation data with comparable data from the aiding sensors 5. The closed-loop error controller 3 then corrects the inertial navigator 2 to achieve a navigation accuracy improvement over what an unaided inertial navigator would be capable of achieving.
The Kalman filter 4 implements a recursive minimum-variance estimation algorithm that computes an estimate of a state vector based on constructed measurements. The measurements typically comprise computed differences between the inertial navigation solution elements and corresponding data elements from the aiding sensors. For example, an inertial-GNSS position measurement comprises the differences in the latitudes, longitudes and altitudes respectively computed by the inertial navigator and a GNSS receiver. The true positions cancel in the differences, so that the differences in the position errors remain. A Kalman filter designed for integration of an INS and aiding sensors will typically estimate the errors in the INS and aiding sensors. The INS errors typically comprise the following: inertial North, East and Down position errors; inertial North, East and Down velocity errors; inertial platform misalignment errors; accelerometer biases; and gyro biases. Aiding sensor errors can include the following: GNSS North, East and Down position errors; GNSS carrier phase ambiguities; and DMI scale factor error.
The error controller 3 computes a vector of resets from the INS error estimates generated by the Kalman filter and applies these to the inertial navigator integration processes, thereby regulating the inertial navigator errors in a closed-loop error control mechanization. This method of INS error control causes the inertial navigator errors to be continuously regulated and hence maintained at significantly smaller magnitudes than an uncontrolled or free-inertial navigator would be capable of achieving.
Kinematic ambiguity resolution (KAR) satellite navigation is a technique used in applications requiring high position accuracy such as land survey and construction and agriculture, based on the use of carrier phase measurements of satellite positioning system signals, where a single reference station provides the real-time corrections with high accuracy. KAR combines the L1 and L2 carrier phases from the rover and reference receivers so as to establish a relative phase interferometry position of the rover antenna with respect to the reference antenna. A coherent L1 or L2 carrier phase observable can be represented as a precise pseudorange scaled by the carrier wavelength and biased by an integer number of unknown cycles known as cycle ambiguities. Differential combinations of carrier phases from the rover and reference receivers result in the cancellation of all common mode range errors except the integer ambiguities. An ambiguity resolution algorithm uses redundant carrier phase observables from the rover and reference receivers, and the known reference antenna position, to estimate and thereby resolve these ambiguities.
Once the integer cycle ambiguities are known, the rover receiver can compute its antenna position with accuracies generally on the order of a few centimeters, provided that the rover and reference antennas are not separated by more than 10 kilometers. This method of precise positioning performed in real-time is commonly referred to as real-time kinematic (RTK) positioning.
The reason for the rover-reference separation constraint is that KAR positioning relies on near exact correlation of atmospheric signal delay errors between the rover and reference receiver observables, so that they cancel in the rover-reference observables combinations (for example, differences between rover and reference observables per satellite). The largest error in carrier-phase positioning solutions is introduced by the ionosphere, a layer of charged gases surrounding the earth. When the signals radiated from the satellites penetrate the ionosphere on their way to the ground-based receivers, they experience delays in their signal travel times and shifts in their carrier phases. A second significant source of error is the troposphere delay. When the signals radiated from the satellites penetrate the troposphere on their way to the ground-based receivers, they experience delays in their signal travel times that are dependent on the temperature, pressure and humidity of the atmosphere along the signal paths. Fast and reliable positioning requires good models of the spatio-temporal correlations of the ionosphere and troposphere to correct for these non-geometric influences.
When the rover-reference separation exceeds 10 kilometers, the atmospheric delay errors become decorrelated and do not cancel exactly. The residual errors can now interfere with the ambiguity resolution process and thereby make correct ambiguity resolution and precise positioning less reliable.
The rover-reference separation constraint has made KAR positioning with a single reference receiver unsuitable for certain mobile positioning applications such as aircraft positioning for conducting aerial surveys. An aircraft on a survey mission will typically exceed this constraint. One solution is to set up multiple reference receivers along the aircraft's intended flight path so that at least one reference receiver falls within a 10 km radius of the aircraft's position. This approach can become time-consuming and expensive if the survey mission covers a large project area.
Network GNSS methods using multiple reference stations of known location allow correction terms to be extracted from the signal measurements. Those corrections can be interpolated to all locations within the network. Network KAR is a technique that can achieve centimeter-level positioning accuracy on large project areas using a network of reference GNSS receivers. This technique operated in real-time is commonly referred to as network RTK. The network KAR algorithm combines the pseudorange and carrier phase observables from the reference receivers as well as their known positions to compute calibrated spatial and temporal models of the ionospheric and tropospheric signal delays over the project area. These calibrated models provide corrections to the observables from the rover receiver, so that the rover receiver can perform reliable ambiguity resolution on combinations of carrier phase observables from the rover and some or all reference receivers. The number of reference receivers required to instrument a large project area is significantly less than what would be required to compute reliable single baseline KAR solutions at any point in the project area. See, for example, U.S. Pat. No. 5,477,458, “Network for Carrier Phase Differential GPS Corrections,” and U.S. Pat. No. 5,899,957, “Carrier Phase Differential GPS Corrections Network”. See also Liwen Dai et al., “Comparison of Interpolation Algorithms in Network-Based GPS Techniques,” Journal of the Institute of Navigation, Vol. 50, No. 4 (Winter 2003-2004) for a comparison of different network GNSS implementations and comparisons of their respective performances.
A virtual reference station (VRS) network method is a particular implementation of a network GNSS method that is characterized by the method by which it computes corrective data for the purpose of rover position accuracy improvement. A VRS network method comprises a VRS observables generator and a single-baseline differential GNSS position generator such as a GNSS receiver with differential GNSS capability. The VRS observables generator has as input data the pseudorange and carrier phase observables on two or more frequencies from N reference receivers, each tracking signals from M GNSS satellites. The VRS observables generator outputs a single set of M pseudorange and carrier phase observables that appear to originate from a virtual reference receiver at a specified position (hereafter called the VRS position) within the boundaries of the network defined by a polygon having all or some of the N reference receivers as vertices. The dominant observables errors comprising a receiver clock error, satellite clock errors, ionospheric and tropospheric signal delay errors and noise all appear to be consistent with the VRS position. The single-baseline differential GNSS position generator implements a single-baseline differential GNSS position algorithm, of which numerous examples have been described in the literature. B. Hofmann-Wellenhof et al., Global Positioning System: Theory and Practice, 5th Edition, 2001 (hereinafter “Hofmann-Wellenhof [2001]”), gives comprehensive descriptions of different methods of differential GNSS position computation, ranging in accuracies from one meter to a few centimeters. The single-baseline differential GNSS position algorithm typically computes differences between the rover and reference receiver observables to cancel atmospheric delay errors and other common mode errors such as orbital and satellite clock errors. The VRS position is usually specified to be close to the roving receiver position so that the actual atmospheric errors in the roving observables approximately cancel the estimated atmospheric errors in the VRS observables in the rover-reference observables differences.
The VRS observables generator computes the synthetic observables at each sampling epoch (typically once per second) from the geometric ranges between the VRS position and the M satellite positions as computed using well-known algorithms such as given in “Navstar GPS Space Segment/Navigation User Interface,” ICD-GPS-200C-005R1, 14 Jan. 2003 (hereinafter “ICD-GPS-200”). It estimates the typical pseudorange and phase errors comprising receiver clock error, satellite clock errors, ionospheric and tropospheric signal delay errors and noise, applicable at the VRS position from the N sets of M observables generated by the reference receivers, and adds these to the synthetic observables.
A network RTK system operated in real time requires each receiver to transmit its observables to a network server computer that computes and transmits the corrections and other relevant data to the rover receiver. The reference receivers plus hardware to assemble and broadcast observables are typically designed for this purpose and are installed specifically for the purpose of implementing the network. Consequently, those receivers are called dedicated (network) reference receivers.
An example of a VRS network is designed and manufactured by Trimble Navigation Limited, of Sunnyvale, Calif. The VRS network as delivered by Trimble includes a number of dedicated reference stations, a VRS server, multiple server-reference receiver bi-directional communication channels, and multiple server-rover bi-directional data communication channels. Each server-rover bi-directional communication channel serves one rover. The reference stations provide their observables to the VRS server via the server-reference receiver bi-directional communication channels. These channels can be implemented by a public network such as the Internet. The bi-directional server-rover communication channels can be radio modems or cellular telephone links, depending on the location of the server with respect to the rover.
The VRS server combines the observables from the dedicated reference receivers to compute a set of synthetic observables at the VRS position and broadcasts these plus the VRS position in a standard differential GNSS (DGNSS) message format, such as RTCM, RTCA or CMR. The synthetic observables are the observables that a reference receiver located at the VRS position would measure. The VRS position is selected to be close to the rover position so that the rover-VRS separation is less than a maximum separation considered acceptable for the application. Consequently, the rover receiver must periodically transmit its approximate position to the VRS server. The main reason for this particular implementation of a real-time network RTK system is compatibility with RTK survey GNSS receivers that are designed to operate with a single reference receiver.
Descriptions of the VRS technique are provided in U.S. Pat. No. 6,324,473 of Eschenbach (hereinafter “Eschenbach”) (see particularly col. 7, line 21 et seq.) and U.S. Patent application publication no. 2005/0064878, of B. O'Meagher (hereinafter “O'Meagher”), which are assigned to Trimble Navigation Limited; and in H. Landau et al., Virtual Reference Stations versus Broadcast Solutions in Network RTK, GNSS 2003 Proceedings, Graz, Austria (2003); each of which is incorporated herein by reference.